By Lyndal Sullivan and Marianne Bate December 2024
Figure 1- Location Map of Red Hill prickly pear pilot study site, ‘Gleniston’ north-east of Mudgee.
In December 2022 a small trial of herbicide treatment of Prickly Pear (Opuntia sp.) was undertaken in the Mudgee area. It pointed to the effectiveness of spraying with Fluroxypyr.
While biological controls were active and limiting the growth of prickly pear, denser thickets were forming in some locations. It appeared an ideal time to trial a range of herbicide treatments that would remove the spreading infestations and individual scattered plants.
The trial was carried out on an actively spreading infestation located on a basalt hill (volcanic plug) known locally as Red Hill, on the property ‘Gleniston’ near Ulan, upper Goulburn River, approximately 50 kms north-east of Mudgee, NSW (Figure 2). The basalt soils support a Grassy White Box Woodland endangered ecological community[1] (480 metres elevation) that had been partially cleared by ringbarking in the 1940-50s. There are occasional frosts.
Prickly Pear was first recorded as a problem in the Mudgee area in the 1920s. The Cactoblastis moth was released in 1953 in the Mudgee area (in the Cudgegong River valley) but considered a failure after 2 years[2]. The moth had been introduced into Australia from Argentina in 1925, and considered responsible for bringing back into production millions of acres of pear-infested land in Queensland and north-western New South Wales in the late 1920’s/early 1930’s.
In the Mudgee area the Cochineal scale insect (Dactylopius coccus) was also released, most recently by the NPWS in the 1980s (landowner pers. comm.). While the biological controls generally helped to suppress the spread in the Goulburn River area some populations were forming larger dense patches particularly on more fertile basalt soils.
Figure 2 – Red Hill March 2022 prior to spraying
Overview of Trial
Four trial plots were set up:
- 2 plots to enable the comparison of spraying with 2 different herbicides: Triclopyr (Garlon/ Apparant) and Fluroxypyr (Starane)
- 2 plots to compare 2 different herbicides for manual / non spray techniques useful for remote work where scattered small groups or individual plants are found: Glyphosate 360 and Aminopyralid/picloram (Vigilant)
A control plot was also established and photographed. Cactoblastis and cochineal scale were still present and active (Figure C).
All plots were treated on 1 December 2022, assessed after 4 months (March 2023), then after 9 months (September 2023), and again after 22 months (October 2024).
Spray Comparisons
Two adjacent plots of 8 sq. metres were pegged out and any fruit on plants removed. Each plot was sprayed using a hand-held sprayer with the following products:
Triclopyr – Apparent Axeman 600 amount used per litre water = 5 ml Apparent Axeman 600 + 5ml Uptake Spray Oil
Fluroxypyr – Starane – 9 ml Starane used per litre water
Each plot of about 8 sq. metres used approx. 3.5 litres of spray taking approximately 15 minutes. Each cladode or “paddle” of each plant was sprayed on both sides where possible.
After almost 4 months (24 March, 2023), the plots were checked and the following observations made:
Triclopyr plot – fruit had formed and new plants had established within the plots
Fluroxypyr plot – no new plants or fruit found, cladodes were yellow
After 9 months (Sept 2023) the Fluroxypyr spray appeared to have been more effective:
- Triclopyr plot – fruit had formed and new plants had established within the plots, most cladodes were yellowing only (Figure 3b)
- Fluroxypyr plot – no new plants or fruit found, cladodes were brown, looking dead (Figures 4a – 4d)
After 22 months (Oct 2024) it was clear that Fluroxypyr spray had been more effective:
- Triclopyr plot – some fruit had formed and there was new growth on cladodes. New plants had established within the plots (Figure 3c)
- Fluroxypyr plot – no fruit or new plants found. All cladodes were brown/ dead looking (Figures 4c & 4d)
- Control plot – flower buds were on cladodes (Figures C1 & C2). Some cladodes showed signs of insect activity / patches of browning off, indicating continued biological control activity
Control
The control plot was untouched by any herbicide treatment methods (Figure C).
Observations in October 2024 indicated a low level of cactoblastis and cochineal activity.
Figure C1 Control Sept 2023. Note the yellowing of cladodes which could be attributed to winter frosts
Hand Treatment Comparisons
Plots of small numbers of plants were identified to make comparisons of 2 different herbicides applied by hand.
- Glyphosate 360 was applied neat using the stab and fill technique. One hole was made in each cladode by a knife or roof punch, then immediately filled with herbicide. For larger cladodes, 2 holes were made (Figures 5a-c).
- Aminopyralid/Picloram (Vigilant) was in a gel form and applied by wiping each cladode using the applicator supplied. Some patches were wiped whilst others were scraped before wiping (Figures 6a-c).
After 4 months the treated plants in the Glyphosate plot looked completely dead, which was not entirely the case for those with Aminopyralid/Picloram (Vigilant) treatment.
After 9 months, a few new plants had emerged in the Glyphosate plot (Figure 5b), but not in the plot treated with Aminopyralid/picloram (Figure 6b).
After 22 months (Figure 5c), new cladodes had emerged although no flowers or fruit, in the plot treated with glyphosate. In comparison, no live cladodes were evident on the plants treated with Aminopyralid/Picloram (Vigilant) gel (Figure 6c). However, as the plots were small, no conclusions could be drawn of differences between wiping or scraping before wiping with Aminopyralid/Picloram (Vigilant).
Glyphosate Treatment plot :
Figure 5a, Figure 5b, Figure 5c
Picloram/Aminopyralid gel Treatment Plot (Vigilant)
Figure 6a, Figure 6b, Figure 6c
Observations from Extensive Spraying – Fluroxypyr
Spraying early summer appeared more effective than spraying in autumn. In March 2023 all prickly pear plants found on Red Hill (outside the pilot plots – called non-trial area) were sprayed with Flyroxypyr (Starane), and all fruit removed. The area of Red Hill covered was approximately 4 ha with scattered patches and individual plants of prickly pear with the infestations making up approximately 25% of the area. Figure 7 shows 3 patches scattered (two in background) and emus that appear to be grazing on the fruit.
Figure 7 – Red Hill before spray March 2023 (non-trial area). The yellowing of cladodes may be attributed to some early frosts.
In September 2023 (after 6 months), the plants were yellowing and any fruit produced was mostly dry (Figure 8a). Live green cladodes were sometimes found under dead ones in dense infestations. Any plants or individual cladodes showing no signs of yellowing were sprayed.
Figure 8a – Red Hill indicator plot. September 2023. Fluroxypyr (non-trial area) 6 months after treatment (Sprayed March 28, 2023)
Figure 8b – Red Hill indicator plot. Oct 15 2024. Fluroxypyr (non-trial area) 18 months after initial spray (March 2023) and 12 months since follow up (Sept 2023)
Findings
Spray Treatment
Where it is not possible to reach each individual plant without touching the ‘spines’, spraying is recommended. Note that ‘spines’ can penetrate clothing and gloves and they are difficult to remove from skin as they do not work their way out.
Fluroxypyr was 100 % effective with 1 thorough treatment showing no regrowth after 22 months. Fruit was removed prior to spraying. Triclopyr was not effective.
Early summer appears to be the most effective time to spray with Fluroxypyr, but this was not rigorously tested. After a cold winter it appears that time to recover is needed for optimal results.
Hand Treatment
Spray treatment is not necessary for small, isolated numbers of prickly pear where it is possible to easily reach all cladodes of each plant without touching the ‘spines.’
For treatment by hand, Vigilant was shown to be more effective. However its major drawbacks are the longer rain-fast period (i.e. 12 hrs cf 6 hrs) and the cost (being currently $154 per litre for refills which is over 11 x more expensive than Glyphosate). Its environmental impact is significantly higher for one of its components (Picloram potassium) with the whole formulated product not having been assessed by the standard tests.
Glyphosate has been found to be effective in the treatment of small isolated plants when applied very thoroughly; with each cladode injected at least once, larger ones twice.
Acknowledgements
Dr Julia Mullins Imrie (personal comments, information on history, area map and editing)
Photos supplied by Lyndal Sullivan
[1] Listed Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) – White box – yellow box – Blakely’s red gum grassy woodlands and derived native grasslands
[2] Muswellbrook Chronicle https://trove.nla.gov.au/newspaper/article/107754962 and Mudgee Guardian, 6 January 1955, p. 13.
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